contributors.sh: only check -by: lines for names

Avoid catching lines that otherwise just says "by:" something.

scripts/contributors.sh

@@ -62,7 +62,7 @@ CURLWWW="${CURLWWW:-../curl-www}"
       git -C "$CURLWWW" log --pretty=full --use-mailmap "$start..HEAD"
     fi
   } | \
-  grep -Eai '(^Author|^Commit|by):' | \
+  grep -Eai '(^Author|^Commit|-by):' | \
   cut -d: -f2- | \
   cut '-d(' -f1 | \
   cut '-d<' -f1 | \